Body weight and risk of oral contraceptive failure*1

OBJECTIVE: To examine the hypothesis that higher body weight increases the risk of oral contraceptive (OC) failure. METHODS: We conducted a retrospective cohort analysis of data from 755 randomly selected female enrollees of Group Health Cooperative of Puget Sound who completed an in-person interview and dietary questionnaire between 1990 and 1994 as control subjects for a case-control study of ovarian cysts. Among the 618 women who were OC ever-users, we used Cox proportional hazards regression models to estimate the relative risk (RR) of pregnancy while using OCs associated with body weight quartile. RESULTS: During 2822 person-years of OC use, 106 confirmed pregnancies occurred (3.8 per 100 person-years of exposure). After controlling for parity, women in the highest body weight quartile (70.5 kg or more) had a significantly increased risk of OC failure (RR 1.6, 95% confidence interval CI 1.1, 2.4) compared with women of lower weight. Higher elevations of risk associated with the highest weight quartile were seen among very low-dose OC users (RR 4.5, 95% CI 1.4, 14.4) and low-dose OC users (RR 2.6, 95% CI 1.2, 5.9), controlling for parity, race, religion, and menstrual cycle regularity. CONCLUSION: Our findings suggest that body habitus may affect metabolism sufficiently to compromise contraceptive effectiveness. Consideration of a woman's weight may be an important element of OC prescription.